The present invention relates to a device for adjusting tension on a seat belt used in a vehicle or automobile, and more particularly, to a mechanical seat belt tension adjuster which can freely set the withdrawing force and the retracting force of the seat belt.
A normal seat belt system currently installed in an automobile has a tension spring employing a leaf spring. The leaf spring is formed in a spiral shape and has an inside end connected to a shaft of a reel for winding the seat belt and an outside end fixed to a housing of a seat belt retractor. The biasing force of the leaf spring exerts torque to the reel in a belt winding direction.
In the aforementioned seat belt retractor, the tension on the seat belt significantly varies with the withdrawn amount of the seat belt. For example, when the seat belt is fully wound up, the distortion of the leaf spring is the minimum and the biasing force is thus weak, so that the tension on the seat belt is thus small. On the other hand, when the seat belt is fully withdrawn, the distortion of the leaf spring is the maximum, the biasing force is thus strong, and the diameter of a circle formed by the seat belt wound on the reel is small. Thus, the tension on the seat belt is large. Due to the variation in the belt tension, the pressure by the seat belt increases against a large occupant to make the occupant uncomfortable. As conventionally pointed out, there is also a problem that the retractor sometimes can not wind up the seat belt completely.
It has been developed to provide a seat belt retractor which can exert a constant tension in spite of the withdrawn amount of the seat belt. For example, proposed by Japanese Unexamined Utility Model Publication No. 58-203772 is a mechanism of equalizing the biasing force of a spring by using cone pulleys. In this mechanism, the biasing force of the spring is amplified and is then transmitted to the reel in a position near the limit to which the seat belt is wound up. On the other hand, the biasing force of the spring is reduced and is then transmitted to the reel in a position near the limit to which the seat belt is withdrawn. In this manner, the tension on the seat belt can be constant in spite of the withdrawn amount of the seat belt.
Belt tension adjusting devices having cone pulleys just like the aforementioned publication are proposed by U.S. Pat. Nos. 5,730,385 and 5,803,400. Although these devices can equalize the biasing force, these devices can not freely set the tension on the seat belt. In addition, although these devices proposed by the patents have switches to change from a light mode to a heavy mode and vice versa, in which the tension values set in the respective modes are significantly different from each other, these devices can not smoothly adjust the magnitude of the tension between a small value and a large value.
In considering the aforementioned problem of the variation in a belt tension as the limit of the mechanical seat belt retractor, there is an idea that a reel for a seat belt is driven by a motor. In this case, the tension on the seat belt can be freely set because the torque of the motor can be freely controlled. Further, there is an idea that the rotational biasing force for the reel is amplified or reduced by using an electromagnet. However, the electric retractor requires a lot of parts as compared to the mechanical retractor using only one leaf spring. The electric retractor also requires wirings for transmitting electrical power and control signals.
The present invention has been made with reference to the aforementioned problems, and an object of the present invention is to provide a mechanical seat belt tension adjuster which can free set the withdrawing force and the winding force of the seat belt.
To solve the aforementioned problems, the present invention provides a seat belt tension adjuster for adjusting the tension on a seat belt used in a vehicle comprising: a reel on which the seat belt is wound; a spring for producing rotational biasing force to be imparted to the reel; and a biasing force adjusting mechanism arranged between the reel and the spring for adjusting the biasing force to be transmitted from the spring to the reel depending on the rotational angular position xcfx89 of the reel. The biasing force adjusting mechanism includes a cam for setting a biasing force adjusting ratio xcex1 to be an arbitrary function depending on the rotational angular position xcfx89 when the biasing force adjusting ratio xcex1 is Fo / Fx, i.e. Fo / Fr=xcex1, wherein Fx is the input biasing force from the spring to the biasing force adjusting mechanism and Fo is the output biasing force from the biasing force adjusting mechanism to the reel.
Since the seat belt tension adjuster of the present invention has the cam for setting the biasing force adjusting ratio xcex1 to be an arbitrary function depending upon the rotational angular position xcfx89 of the reel, the tension (withdrawing force/winding force) of the seat belt can be freely set in spite of variation in the biasing force of the tension spring. Since a retractor of the present invention including the tension adjuster is of a mechanical type, parts including a motor for obtaining external power and wirings are not required.
In the seat belt tension adjuster of the present invention, the spring may be a leaf spring, one end of which is fixed. The biasing force adjusting mechanism comprises a slit plate having an input shaft, to which the other end of the leaf spring is fixed to a position apart from the center of the input shaft, and a slit extending in the radial direction; and a pin slidably situated in the slit. The cam is a cam plate having a cam groove in which the pin slides.
The biasing force adjusting ratio xcex1 can be freely set by suitably selecting the configuration of the cam groove of the cam plate. The seat belt retractor according to the present invention can be compact as compared to a conventional one having cone pulleys.
The seat belt tension adjuster of the present invention may further comprise a reduction gear mechanism arranged between the biasing force adjusting mechanism and the reel for reducing the rotation to be transmitted from the reel to the biasing force adjusting mechanism.
Because of the reduction gear mechanism, the moving amount of the leaf spring can be reduced. Accordingly, there is an advantage of reducing the load of the leaf spring.